Dear Editor: It is known that the cellular cultures provide a useful model for the study of cell-substrate interactions, by reducing the complexity of the in vivo systems. Nevertheless, mammalian chondrocyte cultures are quite rare, because cartilage cells readily lose their phenotypic characteristics when they are grown in vitro (1, 15, 18, 19, 20). Differentiated chondrocytes are highly specialized cells, that synthe-size cartilage specific proteoglycans, ie chondroitin sulfate, and collagens, II, IX and XI, which can accumulate in the direct surroundings of the cell (11, 16). The common observation is that chondrocytes in vitro tend to lose differentiation assuming a fibroblastoid morphology with the concomitant switch from the predomi-nant synthesis of type II to type I collagen (5, 7, 14). In fact, the chondrocyte shape, roundish or polygonal (17, 21) is directly correlated with the synthesis of the cartilage proteins (6, 22, 23). In order to retain the differentiated phenotype, the cartilage cells from vertebrates have been suspended in a tridimensional culture system (4) or have been cultivated in agarose or collagen gels (2, 6, 8, 12, 13). It is known, in fact, that chondrocytes grown on artificial substrates retain differentiation only when the cartilage matrix prevents contact between the cells and the plastic surface (7, 9).

However, the monolayer cultures present several advantages, as the ease of set-up and propagation and the adaptability to both small and large number of cells. Furthermore, the monolayer culture environment can offer a good system for the study of regulatory pathways that play a role in chondrogenesis. For these reasons it is very advantageous to obtain monolayer chondrocyte cultures, actually available from several vertebrates, but not from mouse, that repre-sents an adequate model for the study of bone development in mammals.